Steadying the Span: Horizontal Deflection Limits in UIC 776-3

UIC Leaflet 776-3 Chapter 7, titled “Maximum limits on horizontal bridge deflection,” specifies the permissible transverse deformation values for railway bridges under traffic loads. While vertical deflection is often the primary concern for structural engineers, Chapter 7 focuses exclusively on the lateral movement of the bridge deck, which is critical for maintaining correct track alignment and preventing excessive lateral forces on the rails.

Why Horizontal Deflection Matters

Horizontal deflection in bridges can be caused by wind loads, centrifugal forces from trains on curved tracks, or the “nosing” action of the bogies. If a bridge sways too much sideways, it distorts the track geometry. Chapter 7 ensures that this distortion remains within safe limits to prevent the risk of derailment and to maintain passenger comfort (avoiding sudden “jerks”).

Key Limitations Defined in Chapter 7

The chapter typically imposes limits based on the span length (L) and the maximum speed of the line. Key aspects include:

• Angular Discontinuity: Limiting the angle formed at the expansion joints or ends of the deck to prevent a “kink” in the rails.
• Resonance Avoidance: Ensuring the natural frequency of the bridge’s horizontal vibration does not match the frequency of the passing train’s sway (especially for high-speed lines).
• Maximum Amplitude: The absolute horizontal displacement must often be restricted to a fraction of the span (e.g., L/4000 or specific mm values depending on the track type).

Horizontal vs. Vertical Deflection Limits

Understanding the difference between the focus of Chapter 7 (Horizontal) and Chapter 8 (Vertical) is essential for comprehensive bridge design. The table below illustrates the distinct impacts:

Interaction with Continuous Welded Rail (CWR)

One of the main reasons for the strict limits in Chapter 7 is the use of Continuous Welded Rail. Excessive horizontal movement of the bridge deck can impose severe additional stresses on the rail and its fastenings. If the bridge moves sideways while the rail is fixed, the rail may buckle or snap. Therefore, compliance with Chapter 7 is mandatory to guarantee the lifespan of the track superstructure on bridges.